David Cameron is facing a revolt from Conservative members of the European parliament against the coalition's environmental policies, as they try to scupper a proposed toughening of Europe's climate targets.

The revolt of the MEPs is an embarrassment for the prime minister, who has committed Britain to some of the most ambitious greenhouse gas targets in the world and staked his reputation on leading "the greenest government ever".

Tomorrow the European parliament will vote on whether to toughen the EU's emissions-cutting target from 20% reductions by 2020, compared with 1990 levels, to a 30% cut. The commitment to a 30% cut is in the coalition agreement, and has won support from other member states.

But British Conservative MEPs said they would vote to oppose the 30% cut. The leader of the UK Conservative delegation, Martin Callanan, said: "Conservative MEPs have always been sceptical of the EU unilaterally increasing its target to 30% without a worldwide agreement … European companies will be unable to compete if the reduction targets are set too high.

"Many high energy consuming companies are already being forced to relocate to countries outside the EU, which have little or no environmental legislation, putting many Europeans out of work, and an increased target will exacerbate this trend.

"We are also concerned that the higher carbon costs from an increased target will feed through into energy price increases for domestic consumers, who are already facing steep rises."

His remarks were echoed by several other Tory MEPs who replied to a Guardian question to all Tory MEPs asking how they would vote. Only one out of the 23 – Marina Yannakoudakis – replied to say she would vote in favour of 30%, and Julie Girling said she planned to vote for 20% but might compromise on 25% if that was on the table.

The MEPs' reluctance to agree with the coalition's climate aims is at odds with Cameron's statements. Only last month, after committing the UK to the toughest carbon targets in the EU, Cameron reiterated his green claims: "When the coalition came together last year, we said we wanted this to be the greenest government ever. This is the right approach for Britain if we are to combat climate change, secure our energy supplies for the long-term and seize the economic opportunities that green industries hold … the UK can prove that there need not be a tension between green and growth."

Labour said the vote was tight enough that it could be lost. "Cameron's own MEPs prove that the Conservative party is not serious about tackling climate change. The 'greenest government ever' is increasingly exposed as all talk, and little action," said Meg Hiller, the shadow energy secretary.

"Cameron should step in," said Martyn Williams, a campaigner at Friends of the Earth. "He should tell his MEPs 'we look stupid if you vote against this'."

Downing Street did not respond to a request for comment.

The row among the Conservatives reflects a wider disagreement within the EU over toughening the carbon targets. Although more than 70 large businesses, including Google, Unilever and Scottish and Southern Energy, recently came out in favour of the stiffer cuts, many business lobby groups oppose them.

The move comes as EU environmental policy was undermined by one of the most important figures in the European commission, causing alarm in Brussels. Janusz Lewandowski, the commissioner in charge of fraught negotiations on the future of the EU's €130bn budget, cast doubt on the science of climate change and the future of emissions policy.

In an interview with a Polish newspaper he said: "We already have overambitious agreements on CO2 emission reduction. There is a notion that the thesis that coal energy is the main cause of global warming is highly questionable. Moreover, more and more often there is a question mark put over the whole [issue of] global warming as such."

Lewandowski said the CO2 targets "are too ambitious for the Polish economy … Polish politicians have to persuade that there cannot be a quick jump away from coal. For Poland it would be a disaster."

His remarks were all the more pointed as the Poles at the European council blocked progress on the carbon roadmap to 2050, which the commission hoped would be the basis of a strengthening of climate policy. Poland will hold the revolving presidency of the EU from 1 July.

Ruth Davis, chief policy adviser at Greenpeace UK, said: "It's terrifying that the man in charge of Europe's budget is someone you might expect to see in Sarah Palin's Republican party.

"He has a huge influence over all of our economic futures and yet not only does he deny the overwhelming evidence of climate change, but he's also opposing measures that leading businesses say would drive green growth and create millions of new jobs in Europe's clean industries."

The ructions came as peace unexpectedly broke out in another key area of European environmental policy, when the EU's energy chief and climate chief reached an amicable compromise over a new directive on energy efficiency.

The row was over plans for a new energy efficiency directive from Günther Oettinger, energy commissioner, that will force businesses to cut dramatically the amount of energy they waste. But green experts feared the new rules would create problems for the EU's emissions trading scheme, because if companies reached their efficiency targets they would be left with large amounts of unused carbon permits. The influx of these permits would drive down the cost of carbon overall, and render the emissions trading system useless as a means of driving investment in clean technology, as well as improving efficiency.

Connie Hedegaard, the climate commissioner, favoured setting aside a quantity of permits in order to prop up the carbon price, but many businesses opposed this as they feared it would impose higher costs.

The compromise arrived at on Tuesday means that if the emissions trading system is seen to be failing because of the energy efficiency directive, adjustments can be made – including setting aside permits.